This invention relates to provision of selected sequences of frequencies for spectroscopic analysis or other purposes.
Cavity ring down spectroscopy (CRDS) is a laser-based, high sensitivity, absorption measurement technique that has become competitive with alternative techniques, such as intra-cavity laser absorption spectroscopy, frequency modulation spectroscopy, multipass spectroscopy and photoacoustic spectroscopy, for performing spectroscopic analysis of a sample. CRDS exploits the properties of a high finesse optical resonator, usually formed by two or more high reflectivity mirrors defining an optical cavity, with a sample located in the optical cavity. A narrow bandwidth laser light beam is injected into the optical resonator and abruptly terminated. The resulting rate of decay, R, of light beam intensity is measured and is linearly related to the optical resonator losses by a relation
R=1/xcfx84=Ltot/xcex94trt,xe2x80x83xe2x80x83(1)
Ltot=L(ref1)+L(sample).xe2x80x83xe2x80x83(2)
Here xcfx84 is the ring down decay constant (sec), xcex94trt is the optical cavity round trip time, and L(ref1) and L(sample) are measures of the light beam intensity losses due to the resonator alone (no sample present) and due to sample absorption and scattering, respectively, at the chosen frequency.
Where a sample is to be analyzed using CRDS, a light beam at each of a sequence of two or more frequencies should be provided, in order to adequately characterize the sample by its absorption spectrum or transmission spectrum. In a conventional approach, in order to vary the round trip distance of a light beam within an optical cavity, the cavity is partially disassembled and reassembled by moving and realigning at least one mirror that defines the cavity. This approach is time consuming and inefficient and often does not provide the control needed to accurately provide a pre-selected sequence of resonance frequencies.
What is needed is a technique for providing a sequence of selected central frequencies of a light beam with a narrow frequency bandas measured by the full width at half maximum (FWHM), as an integral part of an optical system used for spectroscopic analysis of materials. Preferably, the technique should be flexible enough to allow provision of a sequence with uniformly spaced frequencies and/or of a sequence with arbitrarily spaced frequencies and should allow the chosen sequence to be stepped through without extensive reconstruction of the optical system. Preferably, the technique should be optically efficient so that substantially all light provided for the optical system is utilized in providing the sequence of light beam frequencies. Preferably, the frequency sequence should be available for use in CRDS analysis or for use in some other process that relies upon a light beam having a sequence of selectable frequencies.
These needs are met by the invention, which uses a cavity ring down approach with a rapidly reconfigurable optical cavity to generate a selected sequence of frequencies within the cavity itself. The distance between a line segment joining two mirrors that form part of the cavity, and a third mirror is varied, by moving the third mirror by a controllable amount. This controllable distance increment is chosen to provide a cavity round trip distance that defines a sequence of two or more selected resonance frequencies. The sequence of selected frequencies can be used to perform CRDS analysis of a sample within the cavity, or to provide a sequence of light beam frequencies external to the cavity for some process that is frequency-dependent.